Analog Input/Output Modules Analog Input Modules To select an analog input module the consider the followings: *Voltage level. *Current input. *Conversion speed. There are two basic types of A/D converter. The first will perform a conversion every 20 ms (the period of the a.c. mains voltage), which gives a good clean reading free from worries of line frequency interference. The second will convert in 2-20 us, giving the possibility of measuring transient data. *At very high rates the PLC may only have time to act as a data logger, storing the data as it is read, and analyzing it some time after the event to report on or display it. Analog Output Modules *The conversion speed of an analog output is generally <100 us and rarely a problem. * Once the resolution of the module is selected we have only to consider the following points: Voltage level. Load resistance. Typically the minimum load resistance is 300 Ohm. Current output. It is often a
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